Place all ingredients in a 16-20 quart stockpot, and cover with 6 quarts of water.
2 (5-pound) whole young roasting chickens, 6 quarts cold water, 3 yellow onions, 1 head garlic, 6 carrots, 4 ribs celery with leaves, 2 tablespoons herbs de Provence, 4 bay leaves, 12 whole cloves, 2 teaspoons whole black peppercorns, 1 tablespoon kosher salt, 1 tablespoon low-sodium soy sauce
Bring to a boil, reduce heat to low and simmer, uncovered, 2½ hours.
Carefully transfer the chickens to two large bowls. Remove the chicken meat from the bones and set aside.
After removing the meat from the bones, place them and the excess broth (from the bones) back into the stockpot. Season to taste – the broth should taste lightly salted.
Continue cooking the broth another 1½ hours.
While the broth reduces, cut or shred the chicken meat and save for other meals. (It freezes beautifully.)
After the stock is finished cooking, strain it over through a colander and discard the remaining solids.
Refrigerate the stock overnight and remove the surface fat the next day.